1. Field of the Invention
The present invention relates to an ink jet printing method for reducing image quality deterioration resulting from a print position shift in printing scan quantities when performing multipass printing using a serial type ink jet printing apparatus.
2. Description of the Related Art
As one example of the printing method using a printing head equipped with a plurality of printing elements for printing dots, there is known an ink jet printing method of printing dots on a print medium by making the printing elements discharge inks. The ink jet printing apparatus uses a printing head in which a plurality of printing elements are arranged. In the printing head, there is a case where variation arises in a discharge amount and a discharge direction among the printing elements, such variations may cause occurrence of density unevenness and stripes in an image.
As a technology for mitigating such density unevenness and stripes, the multipass printing method is known. In a multipass printing system, image data that should be printed in a unit area of the print medium is divided into pieces of image data that are printed by several times of translation (hereinafter referred to as a printing scan) that accompany a plurality of times of printing by the printing head, respectively. Then, by a plurality of printing scans among which conveyance operations intervene, the above-mentioned divided image data is sequentially printed. By doing this, even if a discharge characteristic of an individual printing element includes an irregularity, it does not happen that the dots printed by one printing element exist successively in a scanning direction, and it is possible for an influence of the individual printing element to be dispersed in a wide range. As a result, it is possible to obtain a uniform and smooth image.
When performing the multipass printing as explained above, it will be required to divide an image data for an individual printing scan. Generally, for division like this, it is often the case that a mask pattern in which a printable area (1) that permits printing of the dots and a non-printable area (0) that does not permit printing of the dots are arranged in advance is used. In this case, by conducting a logical AND operation between the binary image data that should be printed in the unit area of the print medium and the above-mentioned mask pattern, the binary image data is divided into pieces of binary image data each of which should be printed in each printing scan.
Generally, in the mask pattern (hereinafter also referred to as only a mask), an arrangement of the printable areas (1) is determined so that the complementary relationship may be built among a plurality of printing scans (or a plurality of printing heads). That is, the printing is configured so that one dot is printed by either one of printing scans on a pixel that is defined as printing (1) by the binary image data. By adopting this configuration, image information before division is made to be saved even after the division.
However, in recent years, density variation and the density unevenness resulting from a shift of a print position (registration) in printing scan quantities or in printing head (nozzle row) quantities have become to be newly considered as problems because of performing the above-mentioned multipass printing. Here, the shift of the print position in printing scan quantities or in nozzle row quantities means a shift between dot groups (planes), such as a shift between the dot group printed by the first printing scan (nozzle row) and the dot group printed by the second printing scan (another nozzle row). The shift between these planes is brought about by a variation of a distance between the print medium and a discharge port plane (head-medium distance), a variation of the amount of conveyance of the print medium, etc. Then, if the shift between planes occurs, coverage of the dots relative to the print medium will vary, which will invite density fluctuation and the density unevenness of an image. Hereafter, as described above, the dot group and a pixel group that are printed by the same printing scan of the same means (i.e., a nozzle row for discharging the same kind of ink) are referred to as a plane.
Normally, the print medium is conveyed by the two roller pairs of a convey roller pair located at an upstream side of conveyance and a print medium discharging roller pair located at a downstream side, but it is known that the density unevenness and the density fluctuation as described above are affected by decentering of these rollers and a change of straddling state of the print medium. Some concrete examples are given below.
(1) In the case where the amount of conveyance of the print medium is not constant even if the amount of rotation of the roller is constant due to the decentering of the roller and slip between the roller and the print medium, the above-mentioned shift between planes occurs periodically.
(2) In a situation of printing a head part and a tail part of the print medium, the straddling reduces to one that is done by one side of the roller pair. Therefore, conveyance at each time becomes unstable or a variation of head-medium distance becomes easy to occur, which makes the shift between planes occur.
(3) When a state where the print medium is straddled only by a conveyance roller pair is switched to a state where it is straddled by two roller pairs of the conveyance roller pair and a print medium discharging roller pair, variations of a conveyance force and a head-medium distance occur suddenly. Therefore, a location where the print medium is sent by a volume larger or smaller than a predetermined conveyance volume arises suddenly and the shift between the planes occurs.
(4) When a state where the print medium is straddled by two roller pairs of the conveyance roller pair and the print medium discharging roller pair is switched to a state where it is straddled only by the print medium discharging roller pair, variations of the conveyance force and the head-medium distance occur suddenly. Therefore, a location where the print medium is sent by a volume larger or smaller than the predetermined conveyance volume arises suddenly and the shift between planes occurs.
Against such a problem, for example, Japanese Patent Laid-Open No. 2002-144637 discloses a technology of reducing a printing ratio in the multipass printing, i.e., a ratio of printable pixels of a mask being used in a location where especially print position shift between planes is easy to occur, such as in the head part and the tail part of the print medium. Since the substantial number of the dots to be printed at the location concerned, i.e., the number of the shifted dots can be reduced by decreasing the ratio of the printable pixels of the mask being used, the print position shift of these dots becomes hard to notice as an image.
However, in the case where a mask whose ratio of printable pixels corresponding to a part of printing scans is reduced is used like Japanese Patent Laid-Open No. 2002-144637, there arises a possibility that an original effect of the multipass printing, that is, an effect of obtaining a smooth image by dispersing influences of the individual printing elements is impaired. Already explained, this is because, since generally arrangements of printable pixels are in a mutually complementary relationship among a plurality of printing scans, if the printing ratio of a part of printing scans is reduced, a ratio of printable pixels of an other printing scan will increase, and an influence of the printing elements corresponding to this scan become easy to appear in an image. Moreover, since a timing at which the ink is given to the unit area of the print medium by a plurality of printing scans and its quantity will differ in each unit area, the density unevenness accompanying this will also be brought about. Furthermore, increasing the printing ratio of a part of the printing head becomes a cause of shortening the life of the printing head.
Moreover, in Japanese Patent Laid-Open No. 2002-144637, since the above-mentioned special mask is used especially only to the head part and the tail part of the print medium, the apparatus cannot respond to a conveyance shift that occurs suddenly in a central part of the print medium and the like.
That is, also when the print position shift between a plurality of printing scans suddenly occurs, it is a difficult situation to print an image that is uniform and smooth in the whole area of the print medium.